Ed Gillespie has not been seen since he left the party at the Roydon Marina Hotel near Harlow, Essex, in the early hours of December 22.
The 38-year-old, from Luton, worked for equipment testing company Bureau Veritas and was reported missing by relatives after he failed to return home.
Chief Inspector Nick Lee, who is leading the search, said: "We are extremely concerned for Mr Gillespie's welfare.
"He had been attending a company party at the hotel and was due to stay overnight before checking out on Saturday morning.
"His car and belongings are still at the hotel but there is no sign of Mr Gillespie.
"We want to hear from anyone who might have seen him, possibly walking around outside the hotel in the early hours of Saturday."
Search teams from Essex Police have spent a week scouring the riverbanks and waterways of the River Stort for any signs of clothing.
On Friday, they found Gillespie's spectacles but their efforts to locate him have been hampered by high water levels, Essex Police said.
Yesterday, the search team were joined by specialist volunteers from Essex Lowland Search and Rescue as they continued their search.
Mr Gillespie was last seen wearing blue jeans, a blue and grey small checked long-sleeved top and green trainers.
Anyone with information is urged to contact detectives at Loughton police station on 101.