If a horse comes up lame, no problem no panic, as an experienced equine sitter you just need to be observant enough to spot this because you’re familiar with a horse’s normal movements and gaits. Your next step is to discuss this with the HO. My concern is if the sitter has no equine experience and doesn’t spot lameness….or a puffy lump in a leg. This is quite apart from spotting the early signs of colic which is very dangerous if not caught early.
The HO will most likely ask you to arrange that their vet will visit to rule out a couple of the most common things that it could be. These include ulcers, kissing spine, founder, EPM, HYPP, Lyme, influenza, cushings, sharp teeth, broken teeth, rotten tooth, tooth abscess, knee chips, hairline fractures from head to toe, cancer, pulled muscle, hoof abscess, weak stifle, lice, worms, conformational defect, headaches, sinus infection, bruised sole, ribs or spine out of alignment, poor fitting tack, laminitis, dehydration, vitamin deficiencies, anaemia, infection and strangles.
Whenever we went away back when we owned 5 horses/ponies one or more of them would always throw a shoe so the farrier would need to be called in…. horses can be complicated.
My Appaloosa who I had from a foal, she went on to play polo.
This is quite timely @BonnyinBrighton as I have my first Housesit with a horse this Saturday. I have no experiences with horses at all except for picking the poo in a wheelbarrow for my sister in law’s horse some years ago! The owner knows this and, indeed, in the listing mentioned that the sitter didn’t have to have experience with horses. It’s just for a week.
Please ask them about colic/sweet itch also tendency for laminitis and any hoof problems like thrush etc so that you’re forewarned. Also definitely watch a YT video about the early signs of colic. Plus check all the fencing on arrival and daily walk the perimeter to check for gaps. Good luck 
What’s a YT video?
YouTube 
just type in ‘colic early signs…’
Will do, thank you
Just a thought- check that the farrier isn’t due during your sit period, as he’d expect you to manage/hold the horse whilst he’s trimming or shoeing the horse. I always ask this and check whether the farrier has been paid on account.
I think that’s very unlikely Bonny, I’m only there for a week. An owner wouldn’t expect me to be there for that
I just thought about it reading a thread on the fb group about dog grooming- and who pays and manages this on a long sit. I realised the farrier thing is one of my video chat questions. I think also the more you know about horses the more you ask, it must be the same with all pets though.
YT - Youtube will mean its a video on there