Hotel Sentry

Hotel Sentry turns a spare iPhone into a room guard.

Leave the old phone plugged in on the desk of your hotel room and arm it. It listens for the pressure change a door makes when it opens, the sound of a knock or a handle, and any movement of the phone itself. When enough of those add up, your own phone and Apple Watch get an alert — with a photo of the room, taken at that moment.

WHY A SPARE PHONE
That drawer phone already has a barometer, a microphone, an accelerometer and a camera. It is a better sensor than most travel alarms you can buy, and it costs nothing to reuse.

HOW IT DECIDES
Hotel Sentry does not fire on a single noise. It scores signals over a rolling ten-second window:
• Air-pressure blip — the signature of a door opening in a closed room
• Recognised sound — a door, a knock, or speech
• Sound spike above the room's own noise floor
• Vibration of the phone
• The phone being picked up — an instant alert

After you arm it, the app spends a minute learning the room: the air-conditioning rhythm, the background noise level, the resting vibration. That calibration is why weather changes and a humming HVAC do not wake you at 3 a.m., but a door does. Choose Low, Medium or High sensitivity to match the room.

ON YOUR PHONE AND WATCH
• Live status: armed, calibrating, disarmed or offline, with the sentry's battery
• Alerts with a photo, and optionally the ten seconds of sound before the trigger
• Remote controls: arm, disarm, take a photo now, or sound the siren
• A Lock Screen Live Activity and Home Screen widget while the room is guarded
• An offline warning if the room phone is unplugged or switched off
• A reminder if you leave the hotel without arming, and automatic disarming when you come back

THE SENTRY SIDE
• A near-black kiosk screen that hides what the phone is doing
• Disarm with a long press and a PIN — three wrong tries alerts you with a photo
• Optional siren and flashlight strobe as a deterrent
• A setup checklist: charger, volume, Guided Access, and Shortcuts automations that reopen the app by themselves

PRIVACY BY DESIGN
There is no Hotel Sentry server and no account. Your two devices talk through your own private iCloud database. Photos and clips are yours alone — the developer cannot see them, because there is nowhere for them to arrive. No analytics, no tracking, no subscription.

You can also share access with someone on a different Apple Account by showing them a QR code.

WORKS ON
iPhone and iPad (iOS 15 and later, so an iPhone 6s still makes a fine sentry), Apple Watch, and Mac.

FREE TO TRY
Install free on both devices and arm the sentry three times, with everything switched on. After that, a single one-time unlock keeps it arming forever — no subscription, ever. Watching a room someone else is guarding stays free.

WHAT YOU NEED
Two Apple devices signed into the same Apple Account (or a QR invitation), and a charger for the room phone. Hotel Sentry is a helpful extra pair of eyes, not a replacement for a hotel safe, a door lock, or common sense.