SHOULD inversion in conditionals

In trying to use inversion with SHOULD to rewrite this sentence "because of the possibility of secondary devices - there were abandonned bags everywhere - arriving ambulance crews were kept back" I elaborate this new sentence "SHOULD SECONDARY DEVICES HAPPEN TO BE IN THE AREA - THERE WERE ABANDONNED BAGS EVERYWHERE - ARRIVING AMBULANCE CREWS WERE KEPT BACK". The question is my native English speaker friends say this sentence is perfectly correct but the grammar guides I have consulted say that SHOULD inversion is only possible when using first conditional sentences. I am very confused. Any ideas?

Your friends are wrong about the grammaticality of the sentence in capital letters.

You could rephrase the bolded sentence as: Should there have been the possibility of secondary devices, arriving ambulance crews would have been kept back. But the original is a factual statement, so a rephrasing with the should conditional changes the meaning.

I am interested to know why you want to use should here.

the grammar guides I have consulted say that SHOULD inversion is only possible when using first conditional sentences.

"Should" inversion as in your sentence is just (a more formal) an alternative to "if" in the protasis . However, the verb in the main clause (apodosis) doesn't have to contain will/going to future form. In other words, the sentence doesn't have to fit the "first conditional" pattern : If X happens, Y will happen. The apodosis as often as not contains a directive. We can use the first conditional in a similar sentence:

Should there be baggage left in the area, the arriving ambulance crews will be kept back.

But we could issue a directive to those in charge of keeping the area clean, like:

Should there be incendiary devices in the area, the arriving ambulance crews are to be kept back.