[Baertram]

HS<number> is a mal formated quick chat message. The game writes them to chat using the |s prefix followed by the number of the quick chat. But as the pipe char is used for itemLinks (which start with h0 or h1, that's why you might see the h at hs) and other chat formatting like |c for color too, some addons need to add support for the pipe s now as the quick chats were added by ZOs with High Isle.

I only wonder why you see that like this, normally the ones sending it cause it to show inproperly at their side. So you might be using any chat addon which causes the parse of incoming chat messages and which causes this formatting issue for you then.
I noticed this at pChat for example and fixed it (I hope I found all occurrences).

Disable your chat related addons 1 after another and check which one causes this. You should be able to test yourself by assigning a quick chat at the help window and use it from the quickslots then (while having the addons enabled).