I love how this topic divolged into a discussion on the definition of stealing..

The Free Dictionary has these definitions:


So I'm pretty confident that you can indeed find something valuable like a bike on the street, if you 'came upon it by chance'. 
If you are stealing it, then depends on what you are doing next. It is required to use some common sense; in the example above, it is clear that the bike belongs to someone, so keeping it would mean 'taking the property of another without permission' and thus be considered stealing.
If it's clear that the bike is somewhere the original owner intends and expects it to be, leave it where it is. If it's clear that it was lost or stolen, probably bring it to some lost-and-found office.

Taking a mushroom from the forest would most probably be considered stealing, indeed. Only because nobody cares about it, that mushroom does belong to someone and does have some value, so after finding it, by taking it home, you stole it.

This doesn't mean that as dkbit98 put it, finding becomes synonymous with stealing.
[1] Finding doesn't imply you keep it (you could return it, for example).
[2] You can also find your own stuff, which obviously would not be stealing.
[3] You could find someone else's stuff and keep it, but have their permission to do so - that would also not be considered stealing, as per the definition above.