Cat poo.
Awoken at 4:30am by a cat emitting strange crying noises, followed by a scampering down the stairs. So I plodded down after her, and let her out, whereupon she trotted up the garden and disappeared through the bushes.
Returning upstairs, the smell hit me. Cat had obviously not been well, and let out some solids, while in distress. "Clean up in aisle 4". Not pleasant at any time, less so when half asleep in the early hours.
Thankfully, she seems fine now, although definitely feeling guilty, and wary of chastisement.