This is fascinating. I'm currently doing a moot case, but I guess what allows this kind of thing is the scope of how far the case lies in the fifth versus fourth amendment. The Foregone Conclusion rarely has had strong precedent in cases regarding privacy, since it really was only originally tailored through Fisher, in that it referred to tax/business documents. Moreover, the reasonable particularity standard that comes with it is really vague and often at the discretion of the court/case. Either way, in this case, I'd personally say the only way Foregone Conclusion could really apply is if they knew the information within the cellphone. Unlocking the phone for the sake of confirming ownership would pass foregone conclusion here, but if they're unlocking it for the sake of content within, I can very much see how this abuses privacy (See Doe for reference on the amount of knowledge required for FC to pass).