Never been to Waverly Hills myself - bit far from Suffolk for a weekend jaunt - but I've had that exact sensation you're describing at several locations over here. That low-level unease, like something's just off but you can't pin it down.
Worth considering a few things before jumping to conclusions though:
Infrasound - old structures generate it constantly, particularly near boiler rooms or collapsed underground areas. It genuinely causes feelings of dread and unease. Has nothing to do with the paranormal but gets misattributed constantly., EMF spikes - caretaker buildings often have older, poorly shielded wiring. High EMF exposure produces anxiety and that ". Being watched". Feeling. Did anyone have a meter on them?
That said, I'm not dismissing you outright. The cottage specifically has come up in reports before, separate from the main sanatorium activity. That's interesting. When multiple people independently flag the same location without prompting, I start paying attention.
What time of day was it? I've found late afternoon at derelict sites often produces more psychological responses than night, oddly enough - something about the light and acoustics shifting.
Anyone else who was there last month, please do chime in. Comparing notes from different visitors is far more useful than any single account. If you had a Mel Meter or similar on you and logged anything, post the readings.