Get hold of a piece of chalk and place it behind your ear. Saunter in after your outing and when she says "Where the F*** have you been?" you say "I met this girl I used to know and we went back to her place and spent the afternoon having fantastic sex"
LL will then spot the chalk and say "Bloody liar! You've been in the pub playing darts haven't you?"
Break down and confess that indeed you nipped in for a pint and got asked to score a darts game.
She'll feel satisfied at having "caught you out" and need never know what you were really doing.
It's worked for me in the past.
