What is hidden in 3D image?

A man and a beautiful woman were having dinner in a fine restaurant. Their waitress, taking another order at a table a few paces away suddenly noticed that the man was slowing sliding down his chair and under the table, but the woman acted unconcerned. The waitress watched as the man slid all the way down his chair and out of sight under the table. Still, the woman dining across from him appeared calm and unruffled, apparently unaware that her dining companion had disappeared.

After the waitress finished taking the order, she came over to the table and said to the woman, "Pardon me, ma'am, but I think your husband just slid under the table." The woman calmly looked up at her and replied firmly, "No he didn't. My husband just walked in the door."