Yes, right when you read the title you probably thought this was just some silly prank someone put up to get people exited about Half life 3. However, this comes right from the Dota 2 program files (at least according to SteamDatabase):
An update to DOTA 2's code has revealed something quite interesting; Half-Life 3 text files. The update was detailed in the SteamDatabase on GitHub and it could either be a hoax or an accidental upload from Valve.
Either way, the text files contain a number of strings that offer commands for some sort of 'Tool Tip' or 'Help' function. The text file is titled 'hl3.txt' and doesn't offer very many exposing details -- but it does mention Hunters, an NPC that first appeared in Half-Life 2.
There are strings for things like "Attribute.npc_hunter_invincible.ignoreunseenenemies" that calls for the Hunter to "Prefer visible enemies, regardless of distance or relationship priority"
On top of references to Hunters, there are also references to Ziplining and NPC's that appear to be in a group with you.
This could, of course, just be some files (or unused files) for half life 2 or source 2 accidentally uploaded to the Dota 2 program files. As Gamezone says, this could also just be a hoax but I think SteamDB is a pretty trustable source.
If you have any opinions comment them here.