I noticed this little plant growing in my two of my pots about 5 months ago. The leaves are 4-6mm in length and about 1mm wide. The plants do flower regularly, however I have not seen them actually open, they are always either bud or spent. I am guessing they are only open for a very shot time or never actually open.
The two pots have plants that I received in a trade and thought thats where they came from. Hoever, they were only growing on the new peat/perlite mix, not from the soil mix that the plants came with (actually bare root, but some mix was still on the roots).
I have come to the conclusion as of today that these have come from seed from my Canadian Peat Moss that I purchased for my planting mixes. The reason for this is that the same little plant is now growing in a pot kept in a fishtank that has never been near the other two larger pots.
I will try to get a photo of the flower buds later, the one I did take didn't come out as it was too dark and blurred.
I am assuming this is a small Utricularia.
Here are some photos of the leaves: