I don't know if the crop field was private property or leased public land, but either way I doubt LE would have needed a search warrant to conduct grid walk searches.
The manhole in question, situated at the edge of the field farthest from Ennis Joslin Road and closest to the Perry Place lift station, was broken. It wasn't simply a matter of the cover being off.
It's helpful, maybe, to imagine a short-necked soda bottle. Picture the cap, neck and shoulder. Per the description offered in the article I've linked below, the manhole was broken back to the shoulder, dislodging its neck, the metal collar in which the cover sits, and the cover itself. In short, at the time LE examined it in April, the manhole was open because it was heavily damaged. There would have been no way to put the cover back on, only rig up something temporary in its place until proper repairs could be made.
The fact that the manhole was broken so extensively - and may have been on March 4 - is the
only reason I can think of for it to have been missed by searchers, mainly because of overgrown weeds, etc., but I still find that just so hard to believe, that it was missed in the first place.
Assistant Chief Todd Green offers a lot more information relevant to the issue of the manhole in this fairly detailed July interview with KIIITV in Corpus Christi.
Manhole, etc.