Cumulative distribution function and recovered flux percentage in the injection procedure for A21-G114.9. Left panel: cumulative distribution function of R(f), normalised to unit area over the interval 5 mJy ≤ f ≤ 1 Jy. The horizontal grey line marks the 95% probability that the bridge in A21-G114.9 has a flux density lower than mJy (green cross), since it is not detected in our observations. This sets an upper limit on its flux density at with a 95% confidence level. With a second criterion based on the extension and continuity of the bridge emission (see Sect. 4.1), we find a lower value for the upper limit at mJy. From the cumulative function, there is an 80% probability that the bridge emission is lower than 125 mJy (red cross). Right panel: plot of the ratio (as a percentage) between the measured flux density and the injected flux density, with a varying injected flux density. The green cross corresponds to the recovered percentage of the 260 mJy injected bridge emission (∼88%), and the red cross corresponds to the recovered percentage of the 125 mJy injected bridge emission (85%). We notice that the fractional recovered flux increases at increasing injected flux density, converging around ∼92%. The injected flux density lost in this procedure is never higher than ∼23%.