I have a 3 level townhouse with the bottom level being the garage. I have a water leak that looks like it is coming from a 1/2 inch pipe in the garage ceiling just below the kitchen sink. It only is present when I run water in the sink but there is no visible water under the sink in the kitchen. It looks like there is some drain type leak but I am very nervous because it is in the ceiling. I have a plumber coming but want to know how to discuss this intelligently so I don't pay a fortune if not needed. Can anyone help me diagnose this... please help

Sounds like a cracked or corroded fitting in the drain side, under the floor. The 1/2" supply would most likely leak at all times and a drain when running, as indicated by your comments. They will have to cut an access hole in the ceiling to see what it is. Rather than patch the hole after repair, screw a plywood access panel over the hole and that way you can easily get in there if it happens again.

Thank you! Would this leak possibly spread water to large areas of the garage ceiling and cause damage?

Yes indeed, get it fixed ASAP