Thanks for the replies. I’ll avoid using aerosol on my bike from now on in general, that’s a good idea. Especially once I put my new rotor+pads on after they arrive.
I’ve got the WD40 one with the nozzle but still got a couple of Fenwicks cans to go through (I was hoping I could use the WD40 nozzle on them but it doesn’t fit, grr). I might even spray them into a jar & use it that way or with a cloth as suggested.
If there is a leak it’s invisible as I’ve thoroughly inspected the pistons for leaking whilst applying pressure with the lever and the pistons partially extended but there’s none appearing.
They are Shimano brakes, yes, Acera M396 ones on my 2016 Trance 3. I’ve read Shimanos can have iffy seals but my front one has been pretty good otherwise; I’ve got the same brake on the back of my hardtail without issue so far. Plus, I bought a big tub of Shimano brake fluid years ago so kind of want to stay with Shimano just to use it up! :D