Check engine light. 1997 subaru gtb just had new battery and alternator put in today check engine light came on on way home toni

1997 subaru gtb. just had new battery and alternator put in today. check engine light came on on way home tonight, still drove absolutely fine, so very confused. turned off and started again, and light didn't come back on. can anyone tell me what is going on!
Thanks

take it back to who ever done the work on it, they may have caused this.

might take it back tomorrow if all goes well, but i rang and he assured me nothing they did could cause it

which i take with a grain of salt

As you should! You can connect up the "Check connectors" under the dash, black male/female plugs, easy to spot, drivers side near column. Then turn the key on and count the number of Engine Light Flashes, so, say 3 long flashes and 1 short flash, would be code 31, more than one code can be displayed, so carefully go through them. Then Google the result.
Fish.

my subaru would now and then throw the check light on, was usually knock sensors. its only a problem if the check light stays on or starts coming on on a regular basis. if poss, do as above says, then you can see what set it off in the 1st place.

The engine check light is letting you know that there's a problem.
The check light will flash and then the fault is recorded in the ecu and you won't see it flash again unless something is unplugged etc.
What has more than likely happened is the ecu has been reset as the battery was disconnected for more than 10minutes. in this time the capacitors drain in the ecu and memory is lost.
The fault was more than likely there before you had the alternator etc replaced so don't go pointing fingers yet.
Connect the black single wire plugs together and switch the red lights on. (these are located to the left of the steering column coming off the main loom). there will also be a pair of green plug(male and female) these are diagnosis plugs
If by pluging the black plugs together don't show any codes then you might wana try a reset.
1. plug black plugs and green plugs in to each other
2. switch on red lights and push the accelerator hard to the floor for 2seconds then lift acceleratorpedal to halfway point for 2seconds then release and start engine. drive the car at a speed of more than 15km and above 2000rpm.
The engine light will flash accordingly, usually takes less than 1minute
If the car flashes fault codes then fix them.
Perform another reset until faults are cleared then the ecu will re-learn
Disconnect plugs after each reset.