The Bosarges’ $5 million mummy now is in exhibition at the Museum of Natural History in Houston. (source)
I am not familiar with "in exhibition". I would always use "on exhibition". My own research on Google Ngrams and Google Books is inconclusive, showing "in exhibition" being far less common. I am not sure "in exhibition" means exactly the same as "on exhibition".
Do people say "in exhibition"?
No, people do not generally say, "in exhibition". Better alternatives are "on exhibit", or "in an exhibition"
I checked the source you provided, for more context. It reads as follows:
They bought $1 million tapestries and a $5 million Egyptian mummy, now in a Houston museum.
Either of the two suggestions I made above would fit here.
This question deals with differences between exhibit and exhibition.
Cambridge definitely gives be on exhibit as a set phrase (and so do other dictionaries too):
if something is on exhibition, it is in a place where the public can come to look at it:
- All the latest cars are on exhibition at the Motor Show.
Strangely enough, if you insert the indefinite article an, NGram shows that in an exhibition is more common than on an exhibition. It is not excluded that the author of that article might have confused the two. A Google of the expression "in exhibition" finds quite a few instances, but not in the way "on exhibition" is used:
- world champion boxers in exhibition fights
- Interested in Exhibition London?
- Women in Exhibition™ is a global cinema network, created to empower women in the exhibition industry.
It is possible they were thinking of On Exhibition when they were saying In an exhibition. The result: In Exhibition would be not an unusual mistake, to make what you are saying fit what you think it is supposed to look like. It is likely why there is any occurrence of the other phrase.
