The closest thing I get to Easter eggs in professional work is leaving references to people or things I like in testing data. Not anything worthy for a spot on The Easter Egg Archive [1], but more plentiful and certainly better than nothing.
I've done this. A long time ago I did QA for a layout product and a lot of the issues were along the lines of "place this square next to this text in this certain way, see the attached screenshot for what it looks like".
The square could have been anything but I had a very cute picture of my toddler so I used a photo of them instead. So now somewhere in a database there are hundreds of reports with a picture of my kid in them :)
I had a coworker who named all the StringBuffer variables "buffy" (as in the vampire slayer). While being a Buffy fan myself, I found this quite annoying.
[1] https://eeggs.com/items/47205.html