Is there a single word that means "strange object"?

Solution 1:

oddity

: a strange or unusual person or thing

M-W

Solution 2:

curio

A rare, unusual, or intriguing object [ODO]

He found many curios in the abandoned house.

Solution 3:

The word curiosity can also be used for that.

curiosity: [countable] someone or something that is interesting because they are unusual or strange:

a house full of old maps and other curiosities

(Longman)

In your case:

He found many curiosities in the abandoned house.

Solution 4:

Anomaly and its variants could be used here.

Merriam Webster:

something that is unusual or unexpected : something anomalous

He found many [anomolies] in the abandoned house.

Solution 5:

curio

He found many curios in the abandoned house.

curio ˈkjʊərɪəʊ noun a rare, unusual, or intriguing object.
"they had such fun over the wonderful box of curios that Jack had sent from India"

[ODO]