OP, have you checked your Starling PIN when out and about?
What some other posters may not realise is that Starling is a challenger bank (you can only use a mobile app to manage the account) and it has all sorts of features like being able to check your PIN in the app by popping in your app password or using your phone security (e.g. fingerprint scan, Face ID). If you've looked at your PIN on your phone, someone could have seen it over your shoulder, for example, if you were sitting on the bus and someone was in one of the higher seats behind you.
If someone had spotted your PIN, they might have targeted you to swipe the card out of your purse. If you normally put all of your cards in the same purse and don't, for example, sometimes put a card in a bag pocket, it seems unlikely that a stranger would have taken your card - mainly because it's harder to steal one card than it is the entire purse/bag.
Starling lets you have a 'connected card' which is effectively a pre-paid joint card you give to someone to buy stuff for you, if you don't trust them enough for a 'real' joint account or, say, they're a volunteer doing your shopping. The connected card looks the same as the normal debit card, but the debit card holder has to move funds across each time (so the person with the connected card doesn't have unlimited access to the main account funds). It's a bit of a confusing concept if you're only used to traditional banks!
The face of each card is identical.
It sounds like the OP has a normal debit card and a connected card (possibly because she gives this to her partner to do the shopping sometimes, given the justified trust issues). When someone started spending on her debit card, she got the automatic notifications on her phone, she wasn't expecting anyone to be buying stuff for her, and she panicked. When she looked in her purse, she saw a Starling card and assumed it was her debit card, but it was actually the connected card. Easy mistake to make - she would have had to take it out and turn it over to see it was the wrong card.
I completely understand the muddle, and also why the OP is so hard to follow.
It's not impossible that a stranger was able to steal the PIN and the main debit card, but given only one card was taken, I'm afraid it's much more likely that it's someone who knows you, OP.