First there was the trademark for Half-Life 3 discovered on a european trademark site
>Trademarked 29/09/2013
http://oami.europa.eu/CTMOnline/RequestManager/en_Result?listType=1&transition=ResultsDetailed&selectOrderby2=&denoselect=1&selectOrderby=&idappli=12180394&ntmark=&application=CTMOnline&bAdvanced=0&totalFound=null&language=en&deno=&ownerselect=&blimited=&source=search_basic.jsp

Then after some more digging, a trademark for the Half-Life Lambda was found on the same site
>Trademarked 1/07/2013
http://oami.europa.eu/CTMOnline/RequestManager/en_Result?listType=1&transition=ResultsDetailed&selectOrderby2=&denoselect=1&selectOrderby=&idappli=11945961&ntmark=&application=CTMOnline&bAdvanced=0&totalFound=null&language=en&deno=&ownerselect=&blimited=&source=search_basic.jsp#

Looking around on that page, it says that the logo was publicized on the 19/08/2013

Doing some more digging, a post was found
>Posted 19/08/2013
http://kotaku.com/steam-developer-accidentally-trolls-half-life-3-fans-1168581785

THREE THINGS FOUND IN ONE DAY
>THREE