Is there any excuse for girlfriend going out with no underwear?

Accidentally ended up in the same club last night she was on a girls night out and I was with the guys. She was a bit awkward, but then we danced with her and I put my hand up the skirt and there was nothing underneath. At first I thought it was for me then I realise she didn't know I was coming! Is that ok? Why would she not have had a thong on?
Is there any excuse for girlfriend going out with no underwear?
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