What is the difference between a hidden object scene and pixel hunting in an adventure game?
With a pixel hunt you can't see what you're looking for and have to click around with your mouse to find an interactive hotspot.
Usually the "hidden objects" in so-called "hidden object games" (at least in the older ones) are not hidden or are only partially hidden, and you can find them by carefully searching the screen with your eyes. This can still be challenging if the objects are small or dark or blurred or otherwise obscured.
Some of the newer "hidden object games" do have pixel hunts because of having "interactive" hidden object scenes. Objects that require "interaction" before collecting them are usually listed in a different color from the regular objects, and may or may not be pixel hunts. For example "lit candle" might require you to use matches on a candle before you can collect the candle. But a different interaction might require you to open a box before collecting an object inside -- and no way to predict that object would be in a box. You would search the screen with the mouse until the cursor changed to indicate possible interaction. But you have to explore with your cursor, just like with a pixel hunt in an adventure game, to locate the interactive objects that aren't clued.
I prefer the older type of hidden object scenes -- either lists or silhouettes -- where the objects can be found 100% by eyeballing and don't require scrubbing around with your mouse, looking for invisible interactions. Although I don't mind something like "lit candle" where you can see the candle and matches on the screen, there are some newer games that take 4 or 5 interactions to find an object, and the object you're looking for has no obvious connection with the first few interactions, which are all "do it because you can and see what happens."
A discussion of the differences between hidden object scenes and pixel hunts probably deserves its own thread. I expect anyone who regularly plays hidden object games knows the difference.
But I may be in the minority in preferring the older searches that didn't require interactions -- at least not multi-step try-it-and-see-what-happens hidden object searches. I may be in the minority in thinking that the multi-step try-it-and-see-what-happens hidden object searches are more like pixel hunts (and less desirable) than the hidden object searches in the older games.