That's why they invented humbuckers!Some gigs the amp and strat are quiet....some buzzy as hell. Especially middle pickup . And its not the typical hum and touching strings does not stop it.
If I turn on 2 pickups it gets quieter but still there. I have a second strat that has the pickups wired to be turned on in series ( i think thats right) so its like a humbucker . May have to start playing that in some venues. BUT sometimes it is the AMP not the guitar....That's why they invented humbuckers!
But seriously, it can vary a lot....fluorescent lights, dimmers, anywhere there's mains electricity (which is everywhere!) you'll get electromagnetic induction. You can find just the right angle to stand at.
I used to play a club in Niagara Falls that was so bad, I had to leave my single coil guitars at home. Turned out that the whole building had 50 volts in the neutral leg.
In any case, always a good idea to carry a volt meter or DMM to check the voltage.