WIPS is also just as useful if you only have WG APs.https://www.watchguard.com/wgrd-products/access-points/wips
There is a background scanning option which does some scanning using the two radios the outdoor (327x) AP has.
You can read more here:https://www.watchguard.com/help/docs/help-center/en-US/Content/en-US/Wi-Fi-Cloud/twe/about_wips.html
APs with the 3rd built in radio (AP225W, AP325, and AP420) can dedicate that 3rd radio to scanning, but 2 radio units can still scan.
Since the coverage area for a WIPS sensor is much larger, depending on the size of your outdoor area, it may be possible to cover it with the 3 radio APs that are inside close to windows. Speaking with your WatchGuard partner and seeing if it's possible to do a survey of that area may help get more information for you.
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