Squirrels in Plumbing

Forget the chemicals. They don't work.

Don't use any acid or drain cleaner. It'l destroy the plumbers snake. He will know and You'll be billed. You may have a blocked vent or vent pipes. a camera may be needed

drain

I have never heard of animals in a sink drain line. They usually appear in toilets because they can see the light shining through the trap seal down into the pipes and they follow it. If you still have squirrels in the pipe, why would they NOT charge you, assuming it was draining properly when they left. It it was not, then you should not have paid them in the first place.

If you had a squirrel problem, I would hope you had your plumber put some type of cap or screen to block access to the vent pipe.

Otherwise you can have a new family that moved in! Squirrels are a PITA in attics, I can only imagine in plumbing.

Best of luck.