In message , Sacha
writes

With alternate foliage and 5-loculate capsular fruits it could be
Hibiscus syriacus. (H. syriacus doesn't usually fruit in my neck of the
woods, but there's a plant in the town centre with similar looking
fruits, that if I recall correctly is H. syriacus.)

You don't happen to remember what the foliage or flowers look like?

For a selection of pictures of H. syriacus see

http://www.malvaceae.info/Genera/Hibiscus/galleryH.html

We think the pods are too pointed for that and Ray suggests
Pseudocamellia stewartia in which the seed pod is very hard. If it's
Fremontodendron we'd expect it to have foliage on still but you never
know! Their seed pods are furry and quite soft.

I don't know Pseudocamellia stewartia. I'm interpreting the photo as
showing a capsule with 5 locules which have separated, so the capsule as
a whole is quite blunt. I would also interpret the capsule as being
chartaceous (parchment-like) rather than woody. A photograph of the
innards of the capsule might assist identification.

I'm sure that it's not Fremontodendron - there's no sign of the fuzz on
the fruit, and I think the OP would have noticed it elsewhere. I never
got seed out of a Fremontodendron, but the mature fruits of
Fremontodendron are quite hard. Also to describe the Fremontodendron as
furry might lead people astray - the seed hairs are quite stiff -
they're not as bad as cacti, at least not being hooked, but they're
quite capable of penetrating skin. The hairs on other parts of the plant
are irritant.

http://www.malvaceae.info/Genera/Fre.../CalGlory.html
--
Stewart Robert Hinsley