Enjoying the Stains of Sex

I was staying overnight at a budget hotel whilst on a working trip and late at night I heard a faint rhythmic thumping next door.I realised it was a couple having sex and listening to the creaking bed and their moaning and groaning got me aroused. The man growled as he came and I imagined his spunk spurting up his lover's cunt. I was wanking at the same time and spermed in unimister. The next morning I heard them at it again until he grunted with lust as he refilled her hole with more spunk. A little later, I went to reception to get some teabags and saw the couple leaving their room. She was in her 50s, plumpish with bleach blonde hair, wearing a tight white short dress, showing a visible panty line around her ample arse. She looked a bit tarty and the guy was even older. As they left, they had left their hotel room door open for the cleaners who hadn't arrived on the corridor yet. When the couple were out of sight, I sneaked into their vacated room and went to the unmade bed. I looked at the mattress where her arse would have been and saw what I was looking for - a large damp stain. It thrilled me to know that this had been caused by her leaking cunt and her man's sperm. I sniffed at the stains and my cock began to stiffen. Then I was even more turned on as I noticed something rolled up under the duvet - it was her discarded panties. She must have been wearing them when she had been fucked and had forgotten then in their haste to leave. I examined the gusset of the white silky knickers and saw that it was wet and sticky with spunk.I sniffed the sex soiled panty crotch and could smell sex on the dirty panty panel. I took the panties back to my room and stripped off, pulling on the slag's dirty knickers. I could feel the spunk stains on my balls and by now my dick was rock hard and leaking pre-cum. I had a good wank, finally pressing my cock head in the musky gusset as I added my spunk to his.

So how did she have the panty line then?