It looks as tough it could be a seedling of Lathyrus vernus. Has lovely purple flowers in Spring and seeds prolifically. I have trimmed mine back as it has finished flowering.
If not that then a vetch of some kind such as Vicia sativa which may have been growing in the sweet pea 'crop' of the suppliers. The seeds are identical to sweet pea seeds so don't get removed by the robot sorting machines.
A trowel in the hand is worth a thousand lost under a bush.
Posts
https://www.rhs.org.uk/Plants/9867/Lathyrus-vernus/Details