The grid background effect you’re observing could indeed be created entirely with CSS, especially if no corresponding image file is found in the website’s assets. There are several CSS techniques to achieve a grid-like appearance without using images.
CSS Grid Layout: Using the CSS grid layout, it’s possible to create a grid-style effect. This method involves defining rows and columns directly in the CSS, which can then be styled with borders to resemble a grid.
CSS Background Patterns: CSS can be used to create patterns with repeated linear gradients or similar techniques. By setting a repeating-linear-gradient or repeating-radial-gradient, developers can simulate complex patterns and textures which may look like a grid.
Pseudo-elements: CSS pseudo-elements (::before and ::after) can be used creatively to overlay grid patterns over an element. By positioning these elements absolutely and applying a transparent border or background with a row and column pattern, a grid can be simulated.
CSS Background Size and Position: By setting a background size that divides the element into equal parts, a grid-like effect can be achieved. You can use a combination of colors and transparency to enhance this effect, using background-size and background-position.
SVG in CSS: Another technique involves embedding small SVG codes directly in the CSS file using the background-image property. This approach allows for scalable and high-quality grid patterns without relying on external image files.

To determine the exact method used on a particular site, inspect the CSS rules being applied to the element. Look for uses of background, background-image, background-size, as well as display, and check for any pseudo-elements that might be rendering over the primary content.