The Sun reports that The Undertaker was at the David H Koch Pavilion Hospital for Special Surgery in Manhattan. It was noted that the hospital is one of the best in the country for orthopaedics and rheumatology. The story did not confirm what Taker was there for.
As noted, Undertaker had been delaying hip replacement surgery until he retired. He did undergo hip surgery back in the fall of 2016, but was said to be hurting during his last two matches at The Royal Rumble in January and WrestleMania 33 last month.

 You can read the full story at The Sun by clicking here. Below is a photo of Taker and his wife, Michelle McCool, at the facility: