I’ve had this before with no prior warning or no notices on drone scene of any flights taking place ( it just happened ) literally a few metres away was caught right off guard,
I was just extra vigilant as you say ( well at least after that ), I thought they put in a notice but does not always seem to be the case 
being the military ( but I’ve not delved into it tbh ) I guess they can half do what they like, they can get away with it I guess.
Edit : @Cedric heres one of the places where it kept happening mate,
