In Iraq, the Jihadi's there were not stupid. By exploiting Western sensitivities with religious sites, they would hide weapons in whatever mosque that was sympathetic. Generally it was with the mosques that were not directly involved with gobbling up American tax payer money. With anyone going in or out, they could have fixed site from which to clandestinely distribute information and a central point to arm the neighborhood. And we, silly infidels we were, could not do a damn thing about it. We did end up changing the rules a bit to be able to enter a mosque to look for weapons but these things had to be very carefully handled with the blessing and in conjunction with the local military or police. These organisations were thoroughly infiltrated by Baathist, Al Queda, (remember the halcyon days of those guys?), Jaysh Al Mahdi, and various other opportunist bad guys. It was always a good bet that someone was tipped off prior to the raid.
Now we have these mosques littered all throughout Europe and the Jihadi battlefield has shifted. Not being located in just Iraq or Afghanistan, they are now free to push into whatever direction they wish. In Matt Bracken's very important connect-the-dots piece, Tet Take 2, he details the major factions at play and the rules of the game. If you have not read this, I would highly recommend you do so. It has the cursory background plus where the latest Salafist push into the West will inevitably go.
The latest German weapons seizure was not the first incident of Jihadi's hiding weapons in mosques. There was a bit of a scare with the Turkish weapons shipment in Greece that was supposedly bound for the Sudanese Police, but the details are specious at best. France has also found weapons in their mosques and vowed to close down 100 to 160 mosques in the coming months. I was not able to verify if this actually happened or if it was more "feel good" hot air with no teeth. Similar raids have happened all throughout Europe with varying degrees of success.
We can not prognosticate what will kick off Bracken's Tet Take 2. The muslim world has not been bashful in its repeated attempts at the conquest of Europe and of Western civilization (which means you too). They have learned well the limits of what they can get away with and the cultural loopholes they can exploit. Recently, after getting a bit of egg on their face by attacking their political allies of the American left at a gay club, they have switched instead to asking all future massacres happen to only white people. Because, you know, American sensitivities.
You combine all of this with the knowledge that they have well funded training villages and training camps and you can see we have a recipe for disaster all throughout Christendom. Is every mosque a terrorist hotbed and hiding weapons? Of course not. What we do know is that they are here. They are armed. Their neighbors are not saying a thing. And there is not a damn thing the Justice Department is going to do about it.