To cut a long story short, it was fortunately at home on the floor & kicked under a table, where it was doing a good impression of a piece of carpet. And it appears nothing actually got stolen from my bag despite either a) me having walked down the road with it open for 10+ minutes or b) someone in the bus crowd having opened it as a prelude to stealing from it. But I have nonetheless endured several hours of stress and worry in the time between noticing my phone was missing and getting home to check.
The worst part was that I rang Orange to report the phone as "maybe stolen, don't know yet" and I wanted them to look to see if it had been used between 4 and 7pm today, but apparently it takes a week for the call log to get to Customer Services. So I had to get it temporarily blacklisted just in case, which means that my phone now won't work until it receives the SIM update un-blacklisting it again. Yeah, I could've just left it... but I didn't really want some random stranger making 101 international phonecalls on my credit :/
So I'm now trying to calm down and relax which is difficult with a churny churny stomach and dizzy stress headache, and I can't ring anyone for a chat because my phone doesn't work again yet. Waah.