Partly because people aren't as secretive as they imagine, and partly that given enough pieces of disparate news, anyone can string together an explanation.
In other words, when a noble is vocal and been around for a while, it is often trivial to work out which way they might jump when desperate. Once you can make that link, then you can start collecting the information required. It was rather obvious that PeL would start reaching out to every power they though might be able to assist them. Given Brom's RP'd personality, it was also obvious he would undertake much of that effort himself, and that he might not be that worried about exactly WHERE the assistance came from.
