@Pat-and-Brooke0
BTW, if it was their normal converters, then they would be the same used in all of the EU i.e. Germany, France, Sweden, Italy, Poland, Spain..... etc. And it would mean that if it was due to the converters, that you would get the same problem with all European guests.
I've read something a while back regarding SMART gadgets and instruments and RFID chipped things, and that is regarding the WiFi connection which works on microwave technology. Not covered by officially controlled media, but there have been scientists saying that the increasing microwave technology is something that can screw up wildlife's orientation capabilities. This was in connection with the widespread bird, fish and other deaths. There are also people who suffer immensely from these microwaves.
Still incredibly strange though, that your dog only reacted to the Danish guests.