Those surrounds are sometimes just freestanding. What happens if you try to lift the top section off? Either that or it will be chased into the plaster (doesn't look that way in the photo), in which case just chisel the plaster away.
It's not very heavy (they're hollow inside) so it'll move if it's going to move. No harm in trying to life one end up a bit. As long as you don't get three of your biggest mates to wrench it off the wall!
if indeed the surround is screw fixed?? try running "slowly" up and down the sides, this will [possibly?] give you a guide as to where the screw fixings are ?
What is the hearth made of? and is the surround resting on the hearth ?
As an aside??? It looks "new" and there are several "modern" hollow plaster fire surrounds, what age is the property?
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