The 29-year-old was on his brother Eamonn’s stag weekend in the German city when he vanished in the early hours of February 10.
Friends and relatives last saw him in the Veermaster Bar after which he was identified on CCTV footage from the Baumwall area of Hamburg.
His family and German police have since led a large-scale search in the surrounding area but Mr Colgan has not been found.
Two new sightings were reported last week prompting Eamonn to return.
An update on the Help Find Liam Colgan Facebook page said: “On Thursday last week our hope for a positive outcome was renewed due to two credible sightings being reported in Altona and at the Reeperbahn.
“On the strength of these sightings Liam’s brother Eamonn and his partner Susan will return to Hamburg this week to give fresh impetus to the search.
“One of the sightings provided us with a photograph which, whilst not conclusive, was the decisive factor for Liam’s family to make this trip. Whilst we are not overly optimistic regarding the photo we have received we feel there are similar characteristics between the male in the photo and Liam that need to be investigated.
“We must also consider that Liam has been missing two months now and his appearance will have changed.
“Our assumption is still that Liam may have suffered a head injury and is unaware of who he is. We ask that anyone who thinks they have spotted Liam to try to speak to him, take a photo (if safe to do so) and immediately report your sighting to Hamburg Police by telephone on 040 4289 56789 or 110 and to our Facebook page so that we can follow it up.”
A body found in a river in the Wandsbek area of Hamburg last week has been identified and is not Mr Colgan.
The Colgan family said: “We send our heartfelt condolences to the deceased’s family.”