A spare shadow cost us four and a half hours...
I've got a small true-story of a night that happened to me and some friends about 27 years ago in the UK, and I just wondered what your theories are...
Four of us left the pub just before last orders, so about 22:45. None of us were drunk, we'd just had a relaxed evening out. We walked back to my mate's house (who was with us), a 30 minute journey that uses three roads, the bulk of which is a main road (the A428). We said our goodbyes, and told his mum and her boyfriend (who were with us) that we'd see them shortly. This was something we did three or four times a week, they'd leave before kick-out and drive back, usually beeping and waving as they passed us, and having cuppas ready when we got in.
Except this night. As we were walking along the main road, I noticed a fifth shadow (we were walking four four-abreast, I was on the end and next to me was a large open Green). It was a shadow whose source should've been a person walking next to me. It wasn't caused by different lights as none of the shadows had doubles, and we tested it by waggling our arms. There was nothing exceptional about the shadow, except there was nobody there. It didn't match any of us for size and build, but was completely normal in these dimensions. None of us noticed it arrive, or leave, but it did both. It was with us for at least fifteen minutes.
Then we got in, having walked straight home, the only difference to normal was a pause of maybe 5 minutes while we "investigated" our new friend. My friend's brother, mum, and her boyfriend were all yelling at us "Where the ******* have you been", "We've been worried sick", "I was about to call the police" etc. It was almost 4am, yet we didn't change anything apart from 5 minutes of trying to ascertain who was casting this shadow.
And they drove the route as normal, and swore blind they didn't pass us at any point.