There would be no need to disconnect the battery to change the timing belt,
So that shouldn't make any differance,
More likely they've either left something unplugged, or started it up with something still disconnected,
Which had stored a fault code and needs cleared
If so its an easy mistake for the mechanc to make and easy put right,
But he should have noticed it and cleared it before the car was returned to you,
Easy way to reset most lights is to leave the ignition switched on and disconnect the neg terminal on batterym
Leave for 5 minutes then reconnect
Turn ignition off then start up as normal, it might clear the fault code to save you going back