Context : Most sub-mm emission line studies of galaxies to date have targeted sources with known redshifts where the frequencies of the lines are well constrained .
Recent blind line scans circumvent the spectroscopic redshift requirement , which could represent a selection bias .

Aims : Our aim is to detect emission lines present in continuum oriented observations .
The detection of such lines provides spectroscopic redshift information and yields important properties of the galaxies .

Methods : We perform a search for emission lines in the ALMA observations of five clusters which are part of the Frontier Fields and assess the reliability of our detection .
We additionally investigate plausibility by associating line candidates with detected galaxies in deep near-infrared imaging .

Results : We find 26 significant emission lines candidates , with observed line fluxes between 0.2–4.6 Jy km s ^ { -1 } and velocity dispersions ( FWHM ) of 25–600 km s ^ { -1 } .
Nine of these candidates lie in close proximity to near-infrared sources , boosting their reliability ; in six cases the observed line frequency and strength are consistent with expectations given the photometric redshift and properties of the galaxy counterparts .
We present redshift identifications , magnifications and molecular gas estimates for the galaxies with identified lines .
We show that two of these candidates likely originate from starburst galaxies , one of which is a so-called jellyfish galaxy that is strongly affected by ram pressure stripping , while another two are consistent with being main sequence galaxies based in their depletion times .

Conclusions : This work highlights the degree to which serendipitous emission lines can be discovered in large mosaic continuum observations when deep ancillary data are available .
The low number of high-significance line detections , however , confirms that such surveys are not as optimal as blind line scans .
We stress that Monte Carlo simulations should be used to assess the line detections significances , since using the negative noise suffers from stochasticity and incurs significantly larger uncertainties .