Hot water leak from heating system
The pressure relief valve must be installed in an upward position, i.e. the red cap must be the highest point. In the position shown in the photo (horizontal installation) the valve closing contact zone can easily be jammed by dirt and rust resulting in a leakage. The red cap offers a maintenance/cleaning procedure - the valve can be twisted and in that way lifted for a short time (&lt; 1 sec) to flush/blow the residual dirt out of the critical zone. This should be done regularly.   
Above this valve there is a location which seems to be a place for an automatic air release valve, which has been removed. Since this type of valve is prone for problems/leakage, a manual procedure for air release may be better, i.e. the removal is not a bad idea.
Although air should not be accumulated in that place since the pressure relief valve under 1. does not work well with air instead of water.
If the pressure at the gauge is increasing with every heating cycle, and decreasing between the cycles, the pressure compensating tank (= big red item in the photo, should have a car valve for testing/filling) could be faulty or could have low pressure (rule of thumb ca. 50kPascal = 0.5 bar + x bar, x is height of the system in meter devided by 10 - for a 3 level building ca. 1-2 bar).
The Grundfos pump in the photo should be checked for correct installation position. The pump has - just as the valve in 1. - a horizontal position, which could be incorrect.
If the leakage problem is not solved, the complete system could be slowly destroyed, since every filling procedure resp. topping up inserts new oxygene into the system that promotes corrosion.
If the tank for the domestic hot water has an sacrificial anode, it must be also checked every year. That water system has no connection to the heating water system, but corrosion could also destroy the heat exchanger and other components.
The label of the right most red rectangular item can not be read, but if it is an electric valve for automated refilling, this valve can also cause the problem by not closing properly due to dirt in the heating system.
Less likely is a problem with the (electronic) control system or the pressure input sensor/filling sensor. 
Normally the domestic water system has a higher pressure than the closed heating water loop.