Strange icon next to the dock

In the past few days I've seen a strange box coming up next to the first icon of the dock on some Macs, here at the office. The same box can be found in the cmd+tab menu.

This happens only on some of our Macs, which have the same version of OSX (10.10.2) and the same AppleID - but other Macs with the same OSX and AppleID don't seem to show the same behaviour.

When the cursor is over the icon, it says "From Mac". The only 2 icons that have appeared there are Chrome and Mail. Clicking the Chrome icon opens a webpage on Chrome that someone else in the office opened recently, and that's a little bit creepy.

I tried to google it a while, without success... I'm sure it's an AppleID related issue, but I can't find a name for it, and hence I can't disable it.

What is that, and how can I get rid of it?