[Link to lyrics](http://www.azlyrics.com/lyrics/elliegoulding/lostandfound.html). 

Frankly, it's anybody's guess. Having said that, it appears that from Stanzas 1 and 2 a boy and a girl have had a romp or good time in the countryside under the moon. They probably had sex. 

Then, Stanza 3:  

>Guess I need to run, take me to the bus  
>But don't let me go, no, don't let me on  
>Gotta hold me tight, won't put up a fight  
>Of course I'll stay, I'll stay the night  

This mentions the girl asking the boy to bring her to the bus station because she "needs to run," which means she needs to leave. But then she asks the boy to not let her on the bus. Therefore they spend more time together, doing who knows what, at the bus station, specifically at the lost and found, which can be an isolated place after daylight hours. Whatever the case, the rather seedy location of a bus stop lost and found has become a place to remember for the boy and girl. Plus the phrase rhymes with the preceding line. 

As far as your suggestions regarding the meaning of *spending time* at *the lost and found*, yes, any and all of them are possible. You would have to ask the song writer and or performer and maybe she couldn't even tell you. But to me  it includes celebrating a night together, and that happened to be at the lost and found, and so that will now always be remembered.