It's very difficult to identify from that description. The only thing I can suggest, from the smell, is whether it's something in the mint family. The usual lemon-smelling thing is Lemon Balm, but the red leaves are wrong for that. And indeed the stalklessness would wipe out most of the mint family.

I think your best bet would be to let it carry on growing for another two or three weeks and see if you get anything more definite in shape for identification. Flowers, of course, would be brilliant, since the plant classification system is largely based on flowering parts.

Meanwhile, can you tell us any more about the leaves? What is their overall shape, eg jaggedly oval, jaggedly heart-shaped? 1 cm long, but how wide at the widest part? How many leaves?? ((about) - ie have you just got the first few leaves of a new plant, or is this settling into being a low stalkless plant? Which leaves have the red undersides - is there a pattern to it? ((like it being the oldest leaves which are red)

I think you've got exactly the right approach,, of not pulling something up until you recognise it. Not only do you get a lot of interesting plants, as you've said, you also learn an awful lot about plant identification. In fact, I reckon the best way to learn about gardening is simply to watch what happens and think about it.