I just saw a water mark , L shaped on 1st floor ceiling. Looks like it may be under toilet. Toilet has not overflowed. Where do you think the leak may be coming from?

Anywhere in the bathroom including tub, shower, sink and toilet. Good news, ceiling must be opened and once you tear out the drywall you will have a much better view of what is leaking. Come back after you opened the ceiling, have a flash light and some toilet paper ready.

The water mark may or may not indicate where the actual leak is occurring.
First, I would open the access panel to the tub if you have one and look for signs of water there. If you have a shower I would pull the escutcheon from around the shower arm and with a flashlight look for water signs in the wall, then I would remove the tub or shower valves trim and look there. I would also pull the toilet and make sure that the wax ring is making a good seal.