Only happening with one bathroom sink, no hot water in toilet tank or bathtub cold faucet or kitchen sink? In large buildings hot water pressure is kept slightly higher than cold so that problems like this can be detected and fixed quickly because of costs involved in hot water loss. Ask your manager if the hot water system pressure was worked on or any new single lever sink faucets installed anywhere at the time your problem started. At a guess the water flow you hear is not an external leak but some sort of cross connect fault that is then slowly dumping through a leaking toilet tank or perhaps some system valve that isn't tightly shut. If this problem is stumping the person hired to fix it, your condo is not hiring very competent people.

Some debris released by working on the clog could now be in the faucet assembly. That debris can defeat or damage the seals. I'd remove the faucet cartridge and check for debris and/or seal damage.

It is physically impossible for this problem to be a result of thermal expansion. Even if this faucett is in the near proximity of the water heater the line is contantly full of water and water is not compressible therefore, in order for thermally expanding water to enter that line it would need to first displace the volume of cold water already in the line and there simply is nowhere for the cold water to go. It is possible during long static periods that heat from a water heater can migrate into the cold water line without actually moving any water but generally this would be limited to about 6' vertical or 4' horizontal from the tank. Heat migration can be eliminated by installing "Heat Trap dielectric nipples" on the water heater. One common cause of warm water on a cold water line results from the installation of a hot water instant on recircutor that uses the cold water line as the return path rather than having a dedicated return line. These type of units are commonly sold in the big box stores as a simple DIY installation for recirculating hot water. It is also now a common prsctice to install a small tempering valve on the watercloset supply line to keep the supply water to the watercloset tank at room temp to prevent condensation on the tank walls. If the lav in question shares a common line with the watercloset fill valve and if the tempering valve is improperly installed before the lavatory connection it would account for this problem. It is also possible that the cold water line is run in parallel to a heating duct that is not adequately insulated and the water is being heated from heat radiating from the duct.

TheRipeTomato: just to clarify, you *are* getting enough pressure from those Rubbermaid "fish barrels" to hand water with a wand? I'm confused, sorry--it seems like you're saying you have a 150 gallon barrel that's not providing pressure, but that you're hand watering with the Rubbermaid bins (which look smaller than 150 gallons). Do you have a pump on the Rubbermaids? If so, what exactly do you use and how are you hooking it up? Sorry for all the questions and thanks in advance.