VAGcom and engine check light

[Datsun]

recently the GF's 1.8T A3 threw up an engine check light, took it to a mates and he plugged in his VAGcom and came up with some error code, i don't recall what it was but gave a description along lines of :
air leak between AFM and throttle housing.

at more or less the same time "service" flashed up on the display.

the following day it went in for a cam belt change and we asked the garage (not Audi btw) to remove the "service" indicator because it will be done within the next week or so.

so got car back and the "service" indication was still there but they had reset the engine check fault code, so no engine check light, got the car home and whipped off engine cover and sure enough found a breather hose from rocker cover to inlet had a bloody great split, replaced the pipe with a new item and everything's been fine.

..until today the engine check light came on again, the GF took it to a local garage and the said the original fault has come back.
so the mechanic took it for a test drive, came back ad proclaimed the car felt fine, but when he re-plugged the computer into the car it didn't give any fault codes, and then the engine check light went out (i assume without him resetting it)

firstly why would the fault reappear? (as in check light) despite 5 mins prior giving the same fault as above, and then later not showing up at all?

secondly is it even possible for the check light to go out on its own accord?

[towcestervag]

clean and reset throttle body
check vacuum and breather hoses fitted below inlet manifold