That assumes there is either someone in the house to hear and act on the alarm , or a neighbour who will act on the alarm.
My experience of alarms is that they are next to useless. Our house was burgled a few years ago, It was approx 3pm in the afternoon (daylight) , we were both at work, but neighbours were around. the house was alarmed, and the alarm triggered as it was supposed to. The thieves still took £11k’s worth of stuff. Everyone just ignored the alarm going off .
If they want it badly enough , they will get it. Thats not to say you shouldnt try and make life difficult, but dont presume that your bike is safe because its chained up, and in an alarmed & locked garage.
My bike is in a locked garage, chained with 13mm chain to a ground anchor , to try and deter all but the most well tooled up thief.
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Julians I agree presuming they’re safe as chained and alarmed is foolish. But from a risk perception point of view, a wireless alarm, pretty much completely removes a night time theft, (assuming decent chains and locks used so not a wheel out job!) and in the daytime if your local you can return when mobile rings, or ring a neighbour/police etc
To have £000’s of bikes in an outbuilding with no alarm is falling at the first hurdle IMHO.