The Krillitane had to be around here somewhere. He saw it, flitting in and out of the alleyways, before disappearing here. He didn't see its human form, so he couldn't tell where it had gone. Security gave him looks. He had to blend in, find it.
So! John Smith suddenly had stomach cramps (a catch-all illness for one who rarely got ill), and sat in line to wait for the clinic doctor. Hopefully the monster would foul up, walk wrong, and the Doctor could tell where it had gone.