Reddit mentions of Govee WiFi Door Sensor, Smart Door Motion Sensor Works with Amazon Alexa APP Control, 2 Door Open Alarm and 1 Wi-Fi Gateway (No Sound), No Monthly Fee - Don't Support 5G

Ok, gonna make some assumptions here but the underlying thought process should be somewhat applicable. If the budget doesn't permit a professional security contractor then this is what I would do "on a strict budget" and recognizing that there are features/some level of reliability you ARE giving up in doing this...

Assumption(s):
Your quoted 400,000 square feet is 3 floors high but assuming it's all one big 'warehouse' which computes to 630' x 630'. Obviously, that's a huge fucking amount of space but here's my thought - if you are truly just trying to get something adequate for long enough to get things out of it then it might make sense to do reactive surveillance rather than active surveillance of the site.

Bare bones, you need access to a power source at each place you want to do this but.... you could just put one of these Wyze Cams with a 32 GB microSD Card at (approximately ~$30/camera + card setup) at each point of ingress/egress & set them to recording continuously to the card. Obviously, if someone who breaks in sees the camera and knows how it works, they could grab the camera and/or SD card and you are shit out of luck... you could put it outside of reasonable reach or obscure it from obvious view. Either way, this is JUST a local backup solution. Also, Disclaimer: I'm not certain if you actually need an internet connection to be constantly present for Wyze cams to keep recording/function - they definitely do local storage on a 24 hour rolling basis but - you'll have to do some research here to know for certain.

You could then just get some cheap contact screamer alarms for each doorway and just disable them when people are there working who are SUPPOSED to be there so everyone doesn't go nuts. .


Moving from "slightly above the barebones" if you wanted active connection/active surveillance capabilities you could:

• Set up a consumer wifi mesh network such as google wifi and deploy wifi cameras + wifi contact sensors...
  
  It's extremely hard to know the layout but, if it's a giant open warehouse you might be able to get away with buying less of these nodes than, say, in a house filled with drywall and with all kinds of things impeding line of sight between the wifi pucks. You can get decent deals on these systems at places like Costco or Sam's Club at around $270 for four of them. Frankly, I'd be surprised if you needed more than 12 but even at that amount you're talking about $1200 to get a wifi network blanketing your entire floor plan.
  
  from there, I'd get the same wyzecam and microSD card setup at $40/camera as outlined above + some wyze sensors or some wifi contact sensors like these. at about $20/door and including an alarm in the sensor - definitely do more research than i just did about what might work best for your situation insofar as wifi contact sensor + alarm devices.
  
  My thought here is, this will give you local storage of video 24/7 on the camera itself + uploaded clips of motion detected on the cameras to your phone and saved in the cloud + a physical alarm that also doubles as a push notification sensor when the door/window/whatever is moved that can also go to your phone.
  
  When you are done, the Google Wifi System could be easily repurposed (probably at multiple locations unless you frequently deal with huge square footage like this). That, and your total cost is south of $3000 (at most & if you have a shit load of doors/windows!). If you want higher quality wifi cameras go with Arlo or something like the Blink system but your costs are going to balloon quickly as those cameras are not cheap.
  
  Alright, hopefully that gets you somewhere! good luck, man!