My tuppence worth from a slightly different angle..
I have a company that operates the gates (Ticket Scanning etc..) at various gigs and events around the country.
Regarding the 'cold calling' I would guess that the Revenue turned up at almost every big festival and gig this year and have done for most over the last couple of years. Also, it tends to be pretty much the same bunch of people all of the time.
Fortunately we have quite a good relationship with them but they willl still ask us how many are working for us on the day, a list of names and PPS numbers etc. I know that they do the same with the Security and Stewarding firms that are in attendance and so I would presume that they also throw an eye over anything else that they might see happening. For example I know for a fact that they regularly have a chat with the Hawkers that operate at these festivals and in around The O2 etc.
I wouldn't be in the least bit surprised to find that they were also tackling the musicians, both at festivals and at local gigs. We all know of busy bands/musicians doing covers that are potentially making €600+ a week so why wouldn't they be investigated.