Just a few questions? How long did you own this plant. Was the spots on it for some time? You just bought it and now see these? Could be; Chemical spray, salt or bleach. Bug invasion. Mold or fungus (the worse). After watching it for a while. If spots get worse. I would use a spray fungicide or sulfur dust on it. I might stay the same and would have to put up with it until the plant is big enough to remove the leaf.

Stush knows way more about Sans than I do, but I think a watering or temperature issue is more likely than a pathogen, or is at least the root of a problem if it is a pathogen. I recently learned with a little experiment that heat is the most important element to growing Sans. Your room is too cool for it to grow, so it isn't using any water. Under those conditions, even a 50:50 mix will retain too much moisture, and the "moist zone" of your water meter is almost certainly too wet. I lost 3 of 4 Bantel's Sensation offsets before I learned this lesson. You might check my post titled "what a difference a little heat makes" to see for yourself how heat affects sans, and a cheap way to warm them in a cool room. Different cultivars are easier or harder to grow, also. Those with lots of variegation, e.g., less deep green tissue, don't have as much chlorophyll and therefore grow more slowly and are more susceptible to rot. Bantel's Sensation, Silver Hahnii, and Golden Hahnii are just three common cultivars that are more challenging to grow. If you don't know your cultivar, a photo would help. You might have a particularly tough one. If I encountered the problem you're having, I'd 1st take Stush's advice, but I'd also forego the water meter and let the plant all but dry out. I use a bamboo skewer to test my soil. I stick it all the way down to the bottom of the pot, wait a few seconds, then pull it up. When a Sans isn't actively growing, the skewer should just feel cool to your upper lip or cheek about halfway up the height of the pot. That means there's just barely enough moisture to keep the root hairs from shriveling up. I have a large Sans in less-than-ideal soil and too cool over the winter. To prevent rot, I use a strong stream from a squirt bottle to shoot a few streams right down into the soil every couple months or so (depends on your humidity, etc.). The moisture is quickly dispersed in the soil without me ever having to soak any part of it. When the weather warms up, I'll have to soak the plant in a bucket a few minutes to rehydrate any peat that has become hydrophobic from drying too much, but the squirt method kept it from rotting over the cold, damp winter (70% humidity and 55-65 degrees). Hope those ideas help, and you don't lose your plant.

It may be fungal. Some little black dots will appear in the brown.. What I have done is segregate the plant (a dry place with good air movement) from your other plants and apply Listerine (or rubbing alcohol, maybe even vodka would work) with a Q-tip or cotton ball to those areas and repeat for a few days - if it goes dry and gray, you killed the fungus and you don't have to necessarily (I have left some, but I could be wrong mind you) remove the entire leaf. It stays scarred like that though. There are some articles on anthracnose in Sansevieria, they recommend spraying with a fungicide, copper for example. The two plants I have were from the same vendor/greenhouse, one was in the bathroom and the other in the kitchen - maybe humidity was to blame?

Plant Lady, Seems you have a disease. There are a few that could spell disaster for you. One is Anthracnose of Sansevieria trifasciata. "This pathogen is host specific to Sansevieria and will not infect other crops. This disease first appears as small brown spots that often looked water soaked. As the disease progresses these spots will become increasingly larger and have the ability to completely stretch across the entire leaf. Dr. Norman’s research is showing that preventative sprays of these chemicals are effective at controlling Sansevieria anthracnose: Pageant, Heritage, Torque, Concert, thiophanate methyl, Affirm, Palladium, Disarm, Cease and Milstop. This typically isn’t a crop that needs to be sprayed and might present an opportunity to pick up some extra chemical sales in the south. There is no cure for this disease, prevention only so staying on top of this disease is key." Second is red spider mites. "It is better to use 3 in rotation if you want to eradicate them. Some good ones are Floramite, Avid, Pylon, Forbid, and Akari." You have to inspect the ends very carefully to see if it is mites or even something like meales. Myself, I would buy some cheap mouth wash with alcohol and make sure it does not have fluoride in it, some do. Mix a spoonful of Baking Soda in and shake well and spray down the plants in question. May help.