we have a similar problem in central london with the congestion charge, you get a request from Met police to go to a swarm in regents street etc and when you get there its gone and you have a a bill of £11.50 for entering the central congestion zone ( £25 from 2020 for non EURO 6 diesels and £12.50 for withing the north circular) )
Will the police/GLA wave that charge , no, even though the may have called you out
Play the same game... you have a basic call out charge, say £200, whether the swarm is there when you get there or not, if it is, charge the basic call out charge, plus so much an hour on top.