One of my igs disappeared in a closet in a way similar to yours. He HAD to be nearby but I couldn't find him. Finally I found him between hanging clothes, clinging to s jacket he had climbed up. He was very hard to notice even though I had searched 3 times.
If the sink has a cabinet, you should thoroughly check the toe-space where it meets the floor for any openings he could get into. This is a well-known place for igs to get lost. Once in, they won't come out.